(1) Field of the Invention
The present invention relates to a function-separated composite vessel which is excellent in the preservability, shape-retaining property, easy openability and resistance to the deformation due to reduction of the pressure. More particularly, the present invention relates to a composite vessel comprising a cup-shaped outer member composed of a paper substrate and a flexible seamless cup-shaped inner member composed of a gas-barrier plastic material.
(2) Description of the Prior Art
Paper vessels are widely used as packaging vessels for preserving drinks and other liquid foods, because they have a good shape-retaining property and they can easily be disposed of by incineration or the like. A paper substrate has a very high gas permeability and a water permeability. Accordingly, there is widely adopted a method in which a paper substrate is combined with a material having an excellent gas-barrier property, such as an aluminum foil, and a film of a thermoplastic resin such as polyethylene is applied to the inner and outer surfaces of the laminate.
If the above-mentioned layer structure is adopted, permeation of gas and water through the vessel wall is effectively controlled, but the seam portion of the vessel of this type still involves a fatal defect as described below.
A paper vessel of the above-mentioned type ordinarily has a side seam formed by lap-bonding both the end portions of a sheet, and a cut edge of the paper sheet is inevitably located on the inner side of this side seam and permeation of gas or liquid from this cut edge is caused. A very troublesome operation of covering and protecting the cut edge located on the inner side of the seam is necessary for preventing the above undesirable phenomenon.
In a paper vessel of the above-mentioned type, when a content is hot-filled so as to improve the preservability, cavity deformation of the vessel wall is caused by reduction of the inner pressure due to decrease of the volume of the content and condensation of vapors.